regulator: add support for the stm32-booster

Add support for the 3.3V booster regulator embedded in stm32h7 and stm32mp1
devices, that can be used to supply ADC analog input switches.

This regulator is supplied by vdda. It's controlled by using SYSCFG:
- STM32H7 has a unique register to set/clear the booster enable bit
- STM32MP1 has separate set and clear registers to configure it.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>